Posted
by
timothy
on Saturday September 10, 2011 @03:39PM
from the they-could-have-saved-gemini-for-this dept.

sighted writes "The twin lunar Gravity Recovery and Interior Laboratory (GRAIL) spacecraft lifted off from Cape Canaveral this morning. GRAIL-A is scheduled to reach the moon on New Year's Eve 2011, while GRAIL-B will arrive New Year's Day 2012. The two solar-powered spacecraft will fly in tandem orbits around the moon to measure its gravity field. Lunar explorers hope the mission will answer longstanding questions about the moon 'from crust to core.'"

Unlike the Apollo program missions, which took three days to reach the Moon, GRAIL will make use of a three- to four-month low-energy trans-lunar cruise via the Sun-Earth Lagrange point L1 to reduce fuel requirements, protect instruments and reduce the velocity of the two spacecraft at lunar arrival to help achieve the extremely low 50 km (31 mi) orbits with separation between the spacecraft (arriving 24 hours apart) of 175 to 225 km (109 to 140 mi).
http://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory [wikipedia.org]

The article unfortunately doesn't say this, but these satellites are very similar to the GRACE [wikipedia.org] pair of satellites still orbiting the earth. There are a couple of things that make this different.

1) One of the limitations GRACE has is that the satellites have to orbit pretty high, to stay out of the influence of the Earth's atmosphere. Their orbits are about 300 miles high, and that limits the resolution of the gravity map to dozens of miles at very best. GRAIL will not have that limitation, and I hope they can fly the satellites much lower. The tidal forces of the Earth at the Moon are probably about 100 times stronger than our tides from the Moon, that might limit how low they can fly -- but it should allow a much more precise measurement of the Moon's gravity map than we have of Earth.

2) One of the fascinating things about GRACE, that has proven more exciting than would have thought possible, is that the Earth's gravity is a function of time. GRACE is able to detect when large areas of earth are saturated with water, or changes in ocean currents, from the change in gravity. The Moon probably doesn't change at all. If they do detect changes, though...that would be exciting!